The present invention relates to a CATV terminal used for a CATV system in which a large number of terminals are connected with a center for the transmission of television signals from the center to the terminals. The present invention also relates to video equipment such as a video cassette recorder (VCR), video disk player (VDP) and television receiver.
Generally, a CATV system is constructed to transmit the programs produced in the center and on-air programs to the terminals by way of cables.
In such a CATV system, trunk line amplifiers are incorporated at approximately twenty stages into one trunk line for the transmission of programs to the terminals, and, additionally, line amplifiers are incorporated at two or three stages after the transmission line is branched off from the trunk line. As a result, the terminal equipment at the farthest end of the line receives signals affected by the influence of noise factors (NF) in the amplifiers thus connected to form twenty or more stages on the line.
Moreover, in the CATV system, some of the programs produced in the center are made chargeable, and viewers at the terminals are charged predetermined charges for the reception of those programs.
Therefore, a scrambling operation is performed at the center, which disorganizes television signals so that the chargeable programs cannot be received in a normal condition with an ordinary television reciever, i.e., to prevent the unauthorized reception.
A descrambling means for restoring the scrambled signals to the original signals is incorporated in the CATV terminal of the subscribers who have been permitted to receive the chargeable programs.
Various kinds of systems have hitherto been proposed for such scrambling and descrambling systems. For example, there is a system called the GSS (Gated Sync Suppression) system, in which only the horizontal synchronizing signal portions and the vertical synchronizing signal portions of a video signal are compressed by several decibels to form a scrambled signal so that the horizontal and vertical synchronization cannot be detected by an ordinary television receiver, while the horizontal and vertical synchronizing signal portions of the video signal, so scrambled by their compression by several decibels, can be expanded to the original level at the side of the terminals authorized to receive the chargeable programs. There is another system generally known as the SSS (Sine Sync Suppression) system, in which a video IF signal is AM-modulated (scrambled) with a sine wave in synchronism with the horizontal synchronizing signal, and at the terminal side the transmitted signal is restored to its original signal by AM-demodulating (descrambling) the video IF signal with a sine wave in the reverse phase.
FIGS. 1(a) through 1(e) illustrate the manner in which the scrambling process and the descrambling process are performed by the GSS system mentioned above.
FIG. 1(a) shows an ordinary baseband video signal, and H in the figure represents a horizontal synchronizing signal. FIG. 1(b) shows a state of this video signal as modulated into an intermediate frequency video signal Fv. This intermediate frequency video signal Fv is processed at the transmitting side, i.e., the center, for the compression of the horizontal synchronizing signal portion by, for example, 6 dB or 10 dB, as illustrated in FIG. 1(c).
This compressed portion corresponds to a horizontal blanking interval (HBI) containing the horizontal synchronizing signal H and the color burst signal Cb, and has a period (t.sub.1) of, for example, 12 .mu.sec. And, in the center, a key signal indicating the compression timing is superimposed on an audio FM signal (not illustrated in the figure) and modulated, together with the intermediate frequency video signal Fv, into an RF signal which is transmitted to the terminals.
The terminal equipment, which exists inside the home, extracts the audio FM signal from the received RF signal and then detects the key signal mentioned above in this audio signal. Then, on the basis of the key signal thus detected, the terminal equipment restores the received video signal to their original state by expanding the horizontal synchronizing signal portions and the vertical synchronizing signal portions by approximately 6 dB or 10 dB in correspondence with the compression rate.
The period (t.sub.2) in which the terminal equipment expands the video signal is set at a period somewhat narrower than that of the compression; for example, 10 .mu.sec. This is to ensure the stability of the descrambling circuit and to prevent partial omission of the picture signal portion.
With this setup, the intermediate frequency video signal Fv will be as shown in FIG. 1(d).
The intermediate frequency video signal Fv which has thus been restored in the terminal equipment is processed by AM detection, which converts it into the baseband signal as illustrated in FIG. 1(e). Thereafter, this baseband video signal is subjected to such processes as volume control of the audio signal and superposition onto the video signal, and the resultant signal is modulated again and sent out to the television receiver in the home.
As described above, the terminal equipment in a CATV system receives the video signal affected by the influence of noise factors (NF) of many amplifiers incorporated into the transmission line from the center to the terminal equipment, so that noises are superimposed on the video signal so received and the signal to noise (S/N) ratio of the video signal is thereby deteriorated.
Moreover, at the time of the scrambling operation of the GSS system at the transmitting side, the HBI portion of the intermediate frequency video signal Fv which contains the horizontal synchronizing signal H and the color burst signal Cb is compressed, and, in the descrambling operation of the GSS system at the side of the terminal equipment which receives the compressed signals via the transmission line, the HBI portion of the signal Fv which contains the horizontal synchronizing signal H and the color burst signal Cb is expanded in a manner reverse to the transmitting side. In such an expanding process, the noises in the HBI portion are also expanded, so that the noises on such a portion increase more in comparison with the picture signal portion so that the S/N ratio in the HBI portion is deteriorated by the rate of the expansion, that is, by 6 dB or 10 dB.
Presently, television receivers are in general provided with an AGC circuit for controlling the amplification gain of the receiver in accordance with the voltage for judging the strength of an input radio wave which voltage is generated from the detected composite television signal, lest a change should occur in the contrast of pictures on the screen even if fluctuations occur in the radio wave induced on the antenna by the fading effect, etc. The television receivers as used at present are provided with two kinds of AGC circuits, one being the peak AGC circuit and the other being the keyed AGC circuit.
The peak AGC circuit is a circuit designed to obtain the AGC voltage from the peak value of the synchronizing signal, utilizing the fact that the synchronizing signal level which marks the peak value in the direction of black of the composite television signal maintains a certain constant value provided that intensity of the radio wave in reception remains constant. In practice, a noise eliminating circuit is provided at the preceding stage, lest a change should occur in the gain because of noises mixed into the signal and having an amplitude equal to or in excess of that of the synchronizing signal. For example, the circuit illustrated in FIG. 2 is employed.
On the other hand, the keyed AGC circuit mentioned above is designed to extract and detect only the horizontal synchronizing signal of the composite television signal and to use this detected voltage as the control voltage signal, and, as such, it is not susceptible to the influence of the noises mixed into the composite television signal. The circuit illustrated in FIG. 3, for example, is used.
As it is evident from the illustrated circuits, a capacitor C is used in either type of the AGC circuits to generate the voltage corresponding to the level of the synchronizing signal, and the capacitor C is required to have a capacitance large enough to generate the voltage corresponding to the level of the synchronizing signal even when it is at the maximum level.
Because of this construction, the charging voltage of the capacitor C will change as shown in FIG. 4 under the influence of the large noise which, as described above, is superimposed on the synchronizing signal portion of the video signal in the process of the signal transmission via the cable or in the descrambling process, and the AGC voltage in the AGC circuit will be affected thereby. This change of the AGC voltage for each horizontal scanning line results in the fluctuations of the video signal level, which cause luminance fluctuations in the form of fine streaks running in the horizontal direction on the screen of a television receiver and consequently cause a deterioration of the picture quality.
Moreover, in general, the luminance component of the back porch portion is used as a reference for the luminance of television receiver. Therefore, if large noises are included in the back porch portion in the descrambling process as mentioned above, the reference for luminance fluctuates for each horizontal scanning line, which causes the appearance of dark horizontal lines on the screen of a television receiver, thereby causing a deterioration of the picture quality.
In the foregoing, the description has been made in conjunction with the CATV terminal. From now on, the video equipment is solely dealt with, separately from the CATV terminal. A television signal received with a VCR or VDP is sent out ultimately to a television receiver, and pictures are displayed on the screen of the television receiver. The television receiver itself also reproduces on its screen a signal received with its antenna.
Television receivers are in general provided with an AGC circuit for controlling the amplification gain of the receiver in accordance with the voltage for judging the strength of an input radio wave which voltage is generated from the detected composite television signal, lest a change should occur in the luminance level or contrast of pictures on the screen even if fluctuations occur in the radio wave induced on the antenna by the fading effect, etc. The television receivers as used at present are provided with two kinds of AGC circuits, one being the peak AGC circuit and the other being the keyed AGC circuit.
The peak AGC circuit is a circuit designed to obtain the AGC voltage from the peak value of the synchronizing signal, utilizing the fact that the synchronizing signal level which marks the peak value in the direction of black of the composite television signal maintains a certain constant value provided that intensity of the radio wave in reception remains constant. In practice, a noise eliminating circuit is provided at the preceding stage, lest a change should occur in the gain because of noises mixed into the signal and having an amplitude equal to or in excess of that of the synchronizing signal. For example, the circuit illustrated in FIG. 2 is employed.
On the other hand, the keyed AGC circuit mentioned above is designed to extract and detect only the horizontal synchronizing signal of the composite television signal and to use this detected voltage as the control voltage signal, and, as such, it is not susceptible to the influence of the noises mixed into the composite television signal period. The circuit illustrated in FIG. 3, for example, is used.
Moreover, in general, television receivers employ a circuit commonly known as the pedestal clamping circuit, in which the luminance component of the back porch portion including the color burst signal subsequent to the horizontal synchronizing signal and the portions fore and rear of the color burst signal is used as a reference for the luminance.
As it is evident from the illustrated circuits, a capacitor C is used in either type of the AGC circuits to generate the voltage corresponding to the level of the synchronizing signal, and the capacitor C is required to have a capacitance large enough to generate the voltage corresponding to the level of the synchronizing signal even when it is at the maximum level.
Because of this construction, the charging voltage of the capacitor C will change under the influence of noises as shown in FIG. 4 if the noises are included in the synchronizing signal portion of the video signal produced by demodulating the broadcast wave received with a television receiver, and the AGC voltage in the AGC circuit will be affected thereby. This change in the AGC voltage results in the deterioration of the picture quality because they cause luminance fluctuations in the form of fine streaks running in the horizontal direction on the screen of a television receiver.
Moreover, if large noises are included in the back porch portion, the reference for luminance in the pedestal clamping circuit is fluctuated by the noises, and such fluctuations result in the appearance of dark lateral horizontal lines on the screen of a television receiver, thereby causing a deterioration of the picture quality.